Process for curing adhesive bonds in laminated structures



Patented Aug; 27, 1946 PROCESS FOR CURING ADHESIVE BONDS IN STRUCTURES Horace W. Hall, Newton, Mass.

No Drawing. Application March 8,1943,

Serial No. 478,476

(c1. 154 1ss) 2 Claims.

This invention relates to a process forthecuring of thermo-setting glue lines in laminated structures by liquid heat and the relieving of internal and surface wood strains affecting the glued joints as well as material so glued, and simultaneous impregnation.

One of m main objects is to solve one of the most troublesome problems in plywood construction. That is, the elimination of internal wood strains in glued-up products which warp the product and cause glue failure in the joints dur ing the post seasoning period after gluing when the glue is aging to its final strength and the wood is accustoming itself to its new shape. This is the cause of millions of dollars of waste yearly. Quite equally important is the so-called conditioning of a glued-up laminated wood product so that surface wood strains will not later develop when the product is subjected to changing atmospheric conditions of moisture and heat which heretofore have seriously interfered with the use of glued-up products for outdoor use irrespective of the water insolubility of the glue used. I This improved method overcomes the difficulties encountered in the curing of thermo-setting glues in thick laminated structures by the usual hot-plate method where the heat is transmitted through the wood laminae to the glued joints,

and is especially applicable to laminated struc-' tures for dowels, tool handles, skis, propeller blades or other articles of thicknesses too great for efficient, hot-plate, curative penetration, with relatively narrow glue lines (length is not an important factor in anyprocess). r

A structure is assembled and pressed together with sufficient pressure so that wood meets wood at all points. Higher pressure ma be desirable. Thus glued and pressed together, the assembly is preferably left for about halfan hour at room temperatures during which timethe glue water evaporates from the edges exposed to the air and the exposed glue of the lines becomes somewhat jelled.

The glue assembly while under pressure, is then immersed in a receptacle containing melted paraffin of standard household quality or paraffin of somewhat higher melting point at a temperature of approximately 200 degrees F., but preferably not to exceed 211 degrees F. The assembly remains in this hot material for-a period of about fifteen to thirty minutes, Or until this heat has traveled the entire glue line and partiall cured the glue. 4 I

Materials which require a higher temperature than 21 1 degrees F. to melt them would not be as desirable because temperatures above boiling volatilize part of the water of the glue near the outside edges of the glue line before the heat partially cures the glue. That isthe reason why I prefer a partial jelling of the surplus, squeezed- -211 degrees F. to the point where and for a length of time until the glue becomes stable, hard, irreversibly water insoluble, which is a complete curing of the glue line. Less than that is a partial curing only. The temperature is raised to approximately 250 degrees F. for instance, for most. urea and phenol formaldehyde 0! other high, heat-setting glues. This will boil the water and exclude the air from the surface cells of the wood or other laminated material used, to a greater r lesser extent, dependin upon the nature 'of the material used and will impregnate these cells by the vacuum thus developed within them with the parafiin or other penetrative material which may be desirable to use. Thereafter the assembl is withdrawn from the liquid bath and the pressure removed preferably after cooling.

If it-is desired not to penetrate the wood to any extent or not so deep but what would be removed on the'working of the product, then a relatively non-penetrating substance of higher specific gravity than paraffin such as carnauba wax or other suitable hydro-carbons with proper melting points could be used. However, when chemical catalysts are added to the high heat curing resin glues, temperatures'below boiling (212 degrees F.) will complete the final curing of the glue line in relatively short periods of time, so that paraffin' or other sci-called impregnating materials can safely be used without noticeable impregnation of the material during the glue curing period, a distinct advantage in economic production. It takes higher temperatures than 212 degrees F. to pro- 7 duce any noticeable impregnation of paraflin or other waxes and fora much longer period of time than that necessary forthe setting and curing of the glue joints under my method.

e A method I prefer when curing urea formaldehyde resin glue lines with said catalysts which cure at ordinary factory temperatures of to degrees F. in two or more'hours, or said warmsetting phenol formaldehyde glue lines whichcure at temperatures of degrees F, or higher at varying periods ranging from ten hours at 110 degrees F. to one hour at 200 degrees F., is as follows:

After gluing and pressing the assembly it is immersed in a liquid heat medium such as previously mentioned which is maintained under 212 degrees F., and removed in 15 to 20 minutes if stock has glue lines 3 inches wide or less, and after relativel longer periods if the glue lines are wider (thickness of stock or number of ply in the assembly does not affect the time limit which is the case with the'usual hot plate press method where the heat is applied to the surfaces and has to be conducted through the Wood). During this short period of immersion the glue lines will be completely cured.

If, however, the structure is to be impregnated with the heat medium, like parafiin, the assembly is glued and allowed to set until the catalyst therein has partially cured the surface edges of the glue line. Said assembly is then immersed in a hot liquid vat of impregnating material heated to above 212 degrees F. (preferably about 250 degrees F.) and kept there until completely cured which takes about 15 to 20' minutes for a glue line 3 inches wide or less and relatively longer for wider glue lines and may then be re moved if slight impregnation is desired or left for relatively longer periods depending upon the depth of impregnation desired, (depth of stock or number of plies does not matter).

A. third method of curing glue joints with chemical catalyst setting glues at low temperatures where impregnation of the liquid heat medium may be desired is that used for the high heat setting types of resin glues first described. For products like laminated shovel han dles or other tool handles and products requiring thickness but not great width in the assembly, these methods are of great advantage as the glue lines are narrow; but the thickness is too great to be cured satisfactorily with the hot plate method commonly used. In my above methods the heat is conducted rapidly and laterally along the glue lines and not through the wood which is apoor conductor of heat.

The heating medium not only should be insoluble in water; but should be of a chemical nature that does not enter into chemical relationship with the resin glue nor affect the Water of the glue line and should be solid at atmos pheric conditions and preferably a hydrocarbon of quick heat conductivity. A material such as that is solid at atmospheric temperature; but softens and melts at temperatures of 120 degrees F. and above is suitable. The pressure is sufilcient to prevent the liquid medium itself from penetrating the glue lineor contacting the glued surfaces which would ruin the glue line.

Instead of accomplishing the partial curing step and complete curing step by continuous immersion in one vat, a two dip method is preferred. In the initial dip I immerse the assembly in a heated vat of either non-impregnating or impregnating liquid material, as desired, at a temperature below 212 degrees F. and keep it there until the glue line has partially cured. This may take 15 to 30 minutes; but the time varies depending upon the glue used. Then, without allowing the assembly to cool, the structure, still remaining under pressure, is immersed in another vat of either impregnating or nonimpregnating material as may be desired at highor temperature than 212 degrees R, such as parafiin, where impregnation of the assembly is stances which, for other purposes, could be carried into the wood by the paraffin. This vat is heated to a temperature above 212 degrees F., preferably 250 degrees, so as to complete the curing of the glue and at the same time to boil off the water and exclude the air from the outer wood cells so that external wood strains are relieved and so that impregnation takes place when so desired by the vacuum so .created in the outer wood cells. The water of the glue line having already entered into chemical union with the resinous adhesive by chemical action promoted inthey first immersion will not be boiled off by the higher temperature of the second immersion. This immersion is continued long enough to completely cure the glue line which may take ten minutes or longer. Said liquid material becomes solid in the cells of impregnation after the stock is, cooled and protects the wood from being affected by water, fungus or insect injury depending upon the nature of the material or combination of materials chosen for impregnation when the product is in use which subjects it to outdoor or other adverse conditions. This curing of the glue line is done in a short space of time, usually less'than one-half hour. There is likewise eifected a final hardening and curing of the glue or so-called aging to final strength and relieving of internal wood strains at the glue joints while the product is immersed in the heat medium, which ordinarily takes several days or more after the glue line has been cured by theusual hot plate, hot air or electrical methods now used, but the relieving of external strains as above described in my methods is not effected by these other methods. v

This utilization of the heat of high heat conductive liquids by immersion of a structure therein as aforesaid to effect conduction of heat along a glue line and effect the release of internal wood strains as well as the conditioning of the outer wood cell structure for protection against external damaging agents, is impossible in other methods using hot air, steam, electricity or hot plates for transmission of heat. The heat conduction laterally along the glue line is so rapid that the inner portion of the glue is cured before the liquid medium impregnates to any extent into the wood cells which would not be the case with known treatments for impregnation with wax. It is the only method that can economically prevent the atmospheric conditions at the time and place of gluing from affecting the glue line when said glue lines are being cured. Thus the changes of setting catalysts which is now necessary for satisfactory work with changes in atmospheric conditions of heat and moisture is, in my method, not necessary, guaranteeing thereby uniformity in the making of positive glue bonds. What I claim is:

1. The process of making a laminated article and curing an adhesive bond within the interior of said article comprising applying an adhesive such as resin glue, that is a relatively good conductor of heat, to surfaces of and assembling and pressing together a plurality of laminations of ligno-cellulosic material of relatively poor heat conductivity, immersing the assembly, while pressed together, in a liquid, such as paraffin, under atmospheric pressure; said liquid being inert to said adhesive and sufficiently hot for the heat to cure the glue line of said assembly and the temperature of which exceeds 212 degrees F., and continuing the immersion until the glue line has been cured.

'2. The process of making a laminated article and curing an adhesive bond within the interior of said article, comprising applying an adhesive such as resin glue that is a relatively good conductor of heat to surfaces of and assembling and pressing together a plurality of laminations of firm, ligno-cellulosic material that is a relatively poor conductor of heat, such as wood, immersing the assembly in a liquid inert to said adhesive that is sufficiently hot for the heat to partially cure the glue line of said assembly but below 212 degrees F. and continuing the immersion and maintaining said pressure until the glue line has been partially cured, and then increasing the temperature of said liquid to a point sufficiently hot for the heat to cure said glue line and continuing the immersion and maintaining said pressure until the glue line has been cured.

HORACE W. HALL. 

